58/ 



black, while that of the next year (n") develops in the form of terraces. 

 The process is repeated from year to year (see u"'-u"") until nearly the 

 whole extent of the axis has been attacked by the canker excrescence and 

 dies. Such places, with open wounded surfaces which become wider and 

 wider, are called "open canker." 



The increase in thickness of the overgrowth edges toward the outside 

 is explained by the fact that plastic material, coming from above, from still 

 living, leaved twigs, has to be divided in each successive year over a smaller 

 part of the twig or trunk surface because of the retrogression of the over- 

 grow^th edges and accordingly provides relatively more al)undant food 



Fig-. 133. Open apple canker. 



Fig. 134. Closed apple canker. 



substances for the formation of new parts, in the cambial zone, which is 

 growing shorter and shorter. 



The closed canker (Fig. 134) when completely developed, represents 

 approximately a spherical wood excrescence (u) at times exceeding the 

 diameter 3 or 4 times, knotted and usually completely covered w-ith bark. 

 This wood excrescence is flattened at its tip and deepened in the centre of 

 the upper surface like a funnel (/). In contrast to open canker, this swell- 

 ing covers a much smaller part of the axis bearing it but makes up for its 

 lesser extent in width liy a considerably greater radial elevation, i. e., greater 

 height. 



